This invention relates to a lens supporting assembly for use in a photoprinting machine.
A photoprinting machine is used to print negative images recorded on a negative film onto printing paper through a printing lens by irradiating the negative film with light. A suitable varifocal lens is selected from a plurality of different kinds of varifocal lenses corresponding to different negative film sizes, and set in the exposure unit of the printing machine. Thus, it is necessary to detachably set such a varifocal lens in the exposure unit so that it can be exchanged for a different one.
FIGS. 10 and 11 show a conventional lens supporting assembly used for this purpose. It has a lens support table 40 and a pair of rails 41 mounted on the lens support table 40. Front and rear positioning pins 42, 43 are provided on the inner surface of each rail 41. A leaf spring 44 is provided in front of the front positioning pins 42. Another leaf spring 45 is secured to one of the rails 41 between the front and rear positioning pins 42 and 43.
A lens plate 47 for supporting a varifocal lens 46 for printing is inserted between the pair of rails 41. It has side plates 48 on both sides. Each side plate 48 has a tapered front edge 49 and is formed with a cutout 50 along the rear edge.
The lens plate 47 is inserted between the pair of rails 41 while keeping it inclined front down, and pushed forward while sliding it on and along the positioning pins 42 so that the front ends of the side plates 48 are inserted under the leaf spring 44. By pushing the lens plate 47 further forward, the rear edges of the side plates 48 pass the rear positioning pins 43 and the rear end of the lens plate 47 is lowered until the lens plate regains its horizontal position. In this state, the lens plate 47 is pushed backward, biased by the leaf spring 44, until the cutout 50 engages the rear positioning pins 43.
To remove the lens plate 47, it is pushed forward to disengage the cutouts 50 from the rear positioning pins 43, inclined by raising its rear end, and pulled out rearwardly.
When attaching and detaching the lens plate 47, it has to be slid on the front positioning pins 42, so that the contact portions of the lens plate 47 and the front positioning pins 42 tend to be worn quickly. With the progression of wear, it becomes increasingly difficult to set the lens plate 47 with high positioning accuracy.
When attaching and detaching the lens plate 47, its rear end has to be raised and lowered. This may cause deformation of the lens plate 47, making it difficult to set the lens plate with high positioning accuracy.
An object of this invention is to provide a lens supporting assembly which can support the lens plate with high accuracy over a long period of time by preventing wear between the lens support table and the lens plate.